Many efforts have been made to increase ROM after TKA, but despite the clinical success of TKA in achieving high ROM, little information can be found in the literature regarding the potential association between objective parameters such as ROM and subjective parameters such as patient satisfaction, and feel of the knee. The present study was performed to determine if an association could be found between objective measurements of increased ROM and subjective parameters of ability to complete activities of daily living, pain, feel of the knee and satisfaction with the knee when high degrees of flexion (beyond 110 degrees) are achieved after TKA.
One of the strengths of this study is the use of patients operated on bilaterally. The use of two treatments in the same patient has the advantage that patient-related factors (i.e. BMI, thigh-calf index, pain threshold) are eliminated, thereby improving the quality of analysis. Is it fair to use two different prostheses? In a study on two different prostheses in bilaterally operated patients, the PS and CR TKA were equally preferred among patients – indicating similar outcomes of feel and satisfaction . Also, it was stated that this method could be the only way to detect the potential subtle differences between prostheses – hence we included the not validated “feel” of the prosthesis.
Did we achieve high flexion? In our study we found that the use of a high-flex PS prosthesis resulted in a significantly increased flexion ability of 7 degrees, both active and passive, when compared to a standard CR TKA at 1-year follow-up. Our results support the data presented by Bin et al. and Weeden et al. [2, 3] who found that higher knee flexion can be achieved when using a high-flex design. In the present study we compare a high-flex PS prosthesis to a standard CR prosthesis. To our knowledge no previous study has compared these designs in achieving high flexion and although it may be argued that a PS-design may give better flexion in weight bearing, the primary goal in this study was not to investigate if the use of a high-flex prosthesis would result in an increased postoperative ROM when compared to a standard prosthesis, but to test if a higher postoperative knee ROM would result in an increased patient satisfaction and better overall patient perceived outcomes.
To our knowledge only a few previously published studies have investigated the relationship between physical findings such as ROM and patient-derived factors such as ADL, satisfaction and “feel” of the knee.
Devers et al.  conducted a retrospective study in which 122 TKA’s were divided into 3 groups depending on their passive flexion 1 year post-operatively. Low flexion was defined as flexion < 110 degrees, medium flexion was defined as 110–130 degrees and high flexion as flexion > 130 degrees. When comparing the medium and high flexion group, they found a positive correlation between the degree of postoperative flexion and fulfillment of expectations, “feel” of knee and functional ability. In contrast, patient satisfaction was not influenced by the degree of postoperative flexion in this study. Two other studies performed by Padua et al. and Ritter et al. [17, 18] also found a positive correlation between increased postoperative flexion, both active and passive, and ability to perform activities of daily living. However, no significant correlation between flexion and patient satisfaction and pain was found.
Unlike Devers et al., Padua et al. and Ritter et al. [13, 17, 18] we found no association between the increased postoperative flexion found in the high-flex group and postoperative knee pain, satisfaction with the TKA, “feel” of the knee and the ability to perform activities of daily living. In our study, the postoperative flexion was equal to or exceeded 95 degrees in all TKA’s, and therefore our results are supported by the data published by Miner et al.  who in a prospective randomized study found no relationship between ROM and ability to perform ADL when flexion exceeded 95 degrees.
The results for SF-36 physical and mental component scores found postoperatively in this study are comparable to results obtained in similar studies of health-related quality of life after TKA [20–24], and to normative SF-36 scores found in the Danish population the age of 65–74 years: PCS 48,39 (mean), MCS 58,54 (mean) . Thus, combined outcomes of both knee designs produce outcomes comparable to other designs and to non-operated age-matched persons – again indicating no limitations in either prosthesis for gaining quality of life.
Some methodological limitations to this study should be acknowledged: First, we did not include a joint specific knee score. Instead, we used the SF-36 questionnaire to assess the health related quality of life one year postoperatively. Second, the follow-up period of 12 months was relatively short and we can draw no conclusions about long-term wear and satisfaction. Earlier studies, however, have revealed that ROM and patient satisfaction reaches a plateau beyond 1 year [6, 25, 26], and therefore we believe that our results can be used as a good marker for long-term function and satisfaction.